The invention relates to a capacitively coupled radiofrequency (RF) plasma reactor and to a process for treating at least one substrate in such a reactor. Especially, the present invention applies to a large size capacitive capacitively coupled (RF) plasma reactor.
Often, such a reactor is known as a “capacitive” RF glow discharge reactor, or planar plasma capacitor or parallel plate RF plasma reactor, or as a combination of the above named.
Capacitive RF plasma reactors are typically used for exposing a substrate to the processing action of a glow discharge. Various processes are used to modify the nature of the substrate surface. Depending on the process and in particular the nature of the gas injected in the glow discharge, the substrate properties can be modified (adhesion, wetting), a thin film added (chemical vapor deposition CVD, diode sputtering) or another thin film selectively removed (dry etching).
The table shown below gives a simplified summary of the various processes possibly performed in a low pressure capacitive discharge.
SubstrateIndustrytypeProcessInlet gas natureSemiconductorWafer up toSurface CleaningAr30 cmPECVDSiH4, . . .diameterDry EtchingCF4, SF6, Cl2, . . .AshingO2Disks forPolymerorDiode sputteringAr + othersmemoryglass up toPECVDOrganometallics30 cmSurface activationO2, etc. . . .diameterFlat displayGlass up toSame as forSame as for1.4 msemiconductorssemiconductorsdiagonalWindow paneGlass up toCleaning/activation,Air, Argon -web coater3 m width,Nitriding, polymerMonomer,foil, plasticPECVDNitrogen, . . .or metal
The standard frequency of the radiofrequency generators mostly used in the industry is 13.56 MHz. Such a frequency is allowed for industrial use by international telecommunication regulations. However, lower and higher frequencies were discussed from the pioneering days of plasma capacitor applications. Nowadays, for example for PECVD applications, (plasma enhanced chemical vapor deposition) there is a trend to shift the RF frequency to values higher than 13.56 MHz, the favorite values being 27.12 MHz and 40.68 MHz harmonics of 13.56 MHz).
So, this invention applies to RF frequencies (1 to 100 MHz range), but it is mostly relevant to the case of higher frequencies (above 10 MHz). The invention can even be applied up to the microwave range (several GHz).
An important problem was noted especially if the RF frequency is higher than 13.56 MHz and a large size (surface) substrate is used, in such a way that the reactor size is no more negligible relative to the free space wavelength of the RF electromagnetic wave. Then, the plasma intensity along the reactor can no longer be uniform. Physically, the origin of such a limitation should lie in the fact that the RF wave is distributed according to the beginning of a “standing wave” spatial oscillation within the reactor. Other non uniformities can also occur in a reactor, for example non uniformities induced by the reactive gas provided for the plasma process.